Exploring the universe

How to Classify Stars

There are different types of stars. Stars are usually classified based on their spectral characteristics. There are seven different types O, B, A, F, G, K, and M. Researchers some times use mnemonic to help them rember the order like
Oh Be A Fine Girl (or Guy), Kiss Me




\fg_c6d4ff M\odot 

\fg_c6d4ff L\odot
O Blue > 25,000 K 60 1,400,000
B Blue 11,000-25,000 K 18 20,000
A White 7,500- 1,000 K 3.2 80
F Yellow 6,000-7,500 K 1.7 6
G Yellow 5,000-6,000 K 1.1 1.2
K Orange 3,500-5,000 K 0.8 0.4
M Red < 3,500 K 0.3 0.04

K = Kelvin 1 Kelvin = -273 ° C or -459.67 Fahrenheit, it is the absolute freezing point colder can it not be, atoms would stop moving. Stars are also classified by luminosity which is the amount of energy that a star emits per unit of time. Luminosity is measured in joules per second or watts just like power. It is a measure of the brightness of the star and is usually counted in comparison with the sun \fg_c6d4ff L\odot

The solar mass is a standard unit of mass in astronomy is  denoted \fg_c6d4ff M\odot where \fg_c6d4ff {\odot } is the sun symbol, 

Type Star
Ia luminous supergiants
Ib less luminous supergiants
II bright giants
III normal giants
IV subgiants
V dwarf
VI subdwarf
VII White dwarf

Hertzsprung-Russell diagram

A famous diagram in astronomy is the Hertzsprung-Russell diagram.
The diagram is a plot of luminosity against the temperature of the star.
Both luminosity and temperature are proportional to the star mass only. Therefore stars of different mass will lie on a line this is known as the main sequence. As the stars spectral characteristics is a good indicator of temperature it is used on the horizontal axis of the diagram


Our sun is a main sequence star of class G2V (yellow dwarf), which means it has a medium temperature and normal size.



How do we classify planets

There are different types of planets and different ways to classify planets. One way is to arrange planets after their mass. Gas giant is called Jovian and is a massive planet with a thick atmosphere and a dense liquid core. In our solar system are Jupiter and Saturn Jovians. Neptunians are planets of the same order as Neptune and Uranus. A super-earth (also called Super-terrestrial) is a planet that is significantly larger than Earth but less than Neptune's about 10 times as large as Earth. A terrestrial planet is a planet of the same size as the Earth.

Planets are also arranged after composition: gas, water-gas, stone-water, stone-iron or iron. Or by temperature, warm, habitable or cold planets. Among extrasolar planets there is something called Hot Jupiter, it's a gas giant that orbit near its star and therefore has a very high surface temperature. These planets are the most common extrasolar planets, because they are relatively easy to detect.

There are seven different criteria that the researchers use to classify planets that can contain extraterrestrial life:

  • Earth Similarity Index (ESI) How similar to a planet is Earth on a scale between 0 to 1 where 1 resembles Earth most. ESI is based on the radius of the planet, density, air velocity and surface temperature.

  • Standard Primary Habitability (SPH) how suitable a planet is for vegetation on a scale of 0 to 1 where 1 is most likely to be possible for vegetation. SPH depends on the surface temperature of the planet

  • Habitable Zone Distance (HZD) distance from the center of the star's habitable zone so that -1 is in the outer zone and 1 is in the inner zone.

  • Habitable Zone Composition (HZC) The planet's composition values ​​below -1 mean that the planet consists mostly of iron and values ​​above 1 that the planet consists mostly of gas. Values ​​close to 0 consist mostly of stone-water-iron.

  • Habitable Zone Atmosphere (HZA) is based on the possibility that the planet has a habitable atmosphere, the values ​​below -1 mean that the planet lacks atmosphere and values ​​above 1 that the planet has a very thick atmosphere and is probably a gas planet. Values ​​close to 0 are not necessarily ideal for habitable atmosphere.

  • Planetary Class (pClass) is based on the planet's mass or temperature zone.

  • Habitable Class (hClass) Classifies planets by Temperature Hypopsychroplanets Cooler Than -50 ° C, psychroplanets Cold (-50-0 ° C), Mesoplanets Normal Temperature (0-50 ° C), Thermoplaneter Warm (50-100 ° C), Hyperthermoplanets warmer than 100 ° C.


Discovery methods of exoplanets

Micro lensing

Even massless photons is affected of gravity, because they have both mass and momentum. It is predicted by Einstein's general theory of relativity that the light will bend when passing an heavy object to follow the curvature of spacetime, it is the closest and most natural path for the light to take. This theory was proven with experiment during a total solar eclipse in 1919 by Sir Arthur Eddington. Similar methods are used to detect exoplanets.

By looking on a distant star, let us call that star for Star A. Star A is far away from another star called Star B, but is aligned behind it when observed by telescope from earth. It will lead to a deformation of light in to two distorted images of Star A. If there is an planet orbiting Star B it produces a third image of Star A.

When light from Star A pass on all sides of Star B it produces a ring of light around the object and that phenomena is known as an Einstein ring.

Transit method

Another method of detecting exoplanets is to measure the magnitude of the star over time. When an exoplanet pass in front of the star there will be a small drop in brightness. This drop will occur for every revolution of the planet. The decrease is very small depending on the planet mass, usually between 0.01 percent and 1 percent.

As the star's mass and size can be determined from spectroscopic analys of the observation then also the mass and the distance of the exoplanet can determined.

Radial Velocity Method

Radial velocity method is also known as Doppler spectroscopy or the wobble method. This measurement is done by recording variations in the color of light from the star. Even for a smaller object like an exoplanet orbiting a star, the gravitational influence of that planet can cause the star to move in a tiny circle. That because the star and the planet orbiting a common center of mass.

Light is electromagnetic radiation and has a wavelength, the visible spectrum is between blue light 380 nm and red light about 740 nm. A common example of the doppler effect is when a ambulance is approaching an observer its siren sound higher in pitch and when it is receding it sound lower in pith. That is because the wavelength of the sound is shorter and has a higher frequency when the ambulance approaching and the wavelength is longer and has a lower frequency when it is receding. Same physics law applies when a star moves towards the earth the wavelength is then shorter and the light from the star color spectrum will shift towards blue. When a star moves from the earth the wavelength is longer and the light will shift towards red color.

Pulsar Timing

A pulsar is an neutron star, a very heavy dense and small object that remains after a supernova explosion. A pulsar emit radio waves when they rotate. If a pulsar has an exoplanet it will behave just like a regular star it will move in its own orbit around a common center of mass. Therefore it could be detected comparing the changes of the radio waves wavelengths. With this method it is possible to detect very small exoplanets. Unfortunately these exoplanets won't be habitable because of the intensity of the high energy radiation from the neutron star                                         

Direct Imaging

This methods means that the exoplanets has been detected directly from telescope image. Exoplanets has a very faint light compering to the host star which makes this a very difficult method. As the planets reflect very little star light they are detected  studying their thermal emission instead. It is easier to detect a planet if it is very large and orbiting close to their star so called hot jovians.

ExoplanetHunter Android App Version 2 Beta

A new version of the Android application Exoplanet Hunter has been released. The android application has been ported to the framework React Native developed by facebook.

React Native is a framework for building native mobile apps using JavaScript and this will allow us to easily build cross platform applications for IOS, android and Web. This update is just for Android but future updates including more platforms will come.

The app is available in two languishes English and Swedish. Compered to the old version the user interface has been improved.

For new users not familiar with the old application here comes instructions on how the application is used:

The app has four sections Planet list shows all planets ordered by last discovered  date. Notice that the old search feature is not gone. If you click the settings gear in the upper right corner you will find two options.

Search and Dashboard in the search settings you will be able to change sort order and filter exoplanets by

  • Distance (light years)

  • Planet Mass Class (mercurian, subterran, terran, superterran, neptunian, or jovian)

  • Planet Composition Class (iron, rocky-iron, rocky-water, water-gas, gas)

  • Planet Atmosphere Class (none, metals-rich, hydrogen-rich)

  • Planet habitable zone classification (hot, warm, or cold).

The gear will be red marked if a selection is active and is white marked if it is cleared.

The Dashboard  will contain information number of discovered Exoplanets and number of habitable planets. More detailed statistics regarding exoplanets in this section is planned for in future releases. If you select a planet in the list you will go to an information page about the planet. Notice this text is autogenerated and automaticity fetched from the PHL database. We never manually look for this information online and write about exoplanets.

Read more about PHL exoplanet data catalog by visiting their website at HEC: Data of Potentially Habitable Worlds‎ 

The tab navigation will feature planet information page and the star information page:

If one click the 3D button a simulation for solar system will be shown

This is generated using SVG and this is a feature that also is planned for the website in the future. The system is zoomable and the planets are clickable and will take you to the information page for the planet that is clicked. The red line and blue line marks the beginning and the end of the stars habitable zone where alien life could exist.

The section earth-like planets will take you to categories for different habitable zones. Planet Earth Similarity Index (ESI) is used for identify possible habitable planets (ESI > 0.7).

The constellation page will list all constellations and the solar systems in each constellation clicking on the star will take you to the simulation of that system.

The last section will be the starmap.

On map initiation will begin at declination 37.87 degree and right ascension 18 hours in the search range area of the Kepler telescope between the constellations Lyra, Cygnus and Drago.

Potential habitable exoplanets will be shown on the map and will be clickable and take the user to the solar system simulation. The map will be draggable at initiation. By clicking the gps icon at the right corner will activate the phones sensors. Accelerometer and magnetometer will be used to pinpoint the location of the planet from the phones orientation using the altitude and azimuth angle of the device. The time shown in the clock will be the time reckoned from the motion of the earth  relative to the distant stars. This is called the sidereal time.

This version is still beta and i can be downloaded at google play. Service will be found under the section api and will replace the old not updated service.

About us

Exoplanet hunter is an application that provides the user with information about scientific discoveries about Exoplanets the project consist of a web application and an Android application.

About the author and developer

Göran Bäcklund is a Swedish physicist and software developer. Göran took his master of science from Linne university in Vaxjo in 2011 his major was computational fluid dynamics. He works as a software developer with projects involving applications used for visualization and simulations of scientific weather data. He has a long time interest in cosmology and amateur astronomy. 

visit on linkedin

Project Background

I got the idea to develop an application for exoplanets during an assignment in a cosmology course. The assignment was to investigate if there were any open data API containing information about exoplanets. Later I developed one of my own using the PHL's Exoplanets Catalog. The first API was built with PHP using the Laravel framework and the first Android application was released in June 2013.

The purpose was to build a good portfolio app, that people interested in science could use to increase their knowledge.

Later the project was forgotten until the beginning of 2018 when I decided to port the old app to React native and .Net Core. Because I use these techniques in my work as a software developer.   

I will continue the project in my spare time, and I will develop some new really awesome features. In the future, the app will also support more platforms. Meanwhile, I will update this website with articles about cosmology and astronomy focusing on exoplanets.  

About the API

The data is provided by the Planetary Habitability Laboratory (PHL) the University of Puerto Rico at Arecibo and can be found here: phl.upr.edu 

The API is using OData protocol that allows for logical querying capabilities. E.g if you want to find habitable planets "$filter= Habitable eq true". Find planet with name 14 And b "$filter=Name eq '14 And b’". Read more here: msdn.microsoft.com. Skip and top E.g takes 30 and skips 10 "$top=30 $skip=10". Order by ascending discovery year $orderby=Disc_Year asc.

About the web

The client app is built with the javascript framework react and semantic UI. There are three sections on the site:

  • Catalog a section where you can browse all confirmed Exoplanets. It is possible to filter for just habitable planets or search for a specific planet name. You can visit information pages about the planet and the star and see you can see a 3D simulation of each solar system.
  • Map in this section we have an interactive star map of all the 88 modern constellations. And all habitable planets are shown as markers in the map. The map is using Leaflet
  • Chart  Hertzsprung-Russell diagram the bubble chart is a plot of luminosity against the temperature of the star for all stars that have exoplanets. The charts are using Amchart

About the app

We live in one of the most exciting times ever, hundreds of exoplanets have been found. An extrasolar planet, or exoplanet, is a planet outside our Solar System. Even planets very similar to our own Earth have been confirmed which raises questions about the existence of extraterrestrial life. The purpose of the application is to give the user a comprehensive and accurate overview over discovered exoplanets.

  • Easy to read information pages for all found exoplanets
  • The application shows a 3D simulation of each solar system
  • Search for the planets locations in the sky by using GPS and the sensors of the phone
  • Completely Free!


Network communication: Full network access, Your location: Approximate (network-based) location, mock location sources for testing, precise (GPS) location

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